In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller feature sizes are required. These may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high-resolution photo-lithographic processes as well as high resolution metrology and inspection instruments and systems. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which dissolves either the exposed or unexposed depending on the type of photoresist (i.e., positive or negative resist, thus leaving a photoresist pattern corresponding to the desired pattern on the silicon wafer for further processing.
In addition to lithographic processes, other process steps in the fabrication of semiconductor wafers require higher resolution processing and inspection equipment in order to accommodate ever shrinking feature sizes and spacing. Measurement instruments and systems are used to inspect semiconductor devices in association with manufacturing production line quality control applications as well as with product research and development. The ability to measure and/or view particular features in a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better products, reduce defects, etc. For instance, device measurements of critical dimensions (CDs) and overlay registration may be used to make adjustments in one or more such process steps in order to achieve the desired product quality. Accordingly, various metrology and inspection tools and instruments have been developed to map and record semiconductor device features, such as scanning electron microscopes (SEMs), atomic force microscopes (AFMs), scatterometers, spectroscopic ellipsometers (SEs), and the like. Scatterometers, as used in this context, are optical instruments that employ algorithms to invert the parameters of a grating from the measured optical characteristics. Typically, scatterometers are used to measure gratings with lateral dimensions that are finer than wavelengths employed by the instrument. The fundamental optical instrument for a scatterometer may be identical to optical instruments used, e.g., for thin-film metrology. Thus an SE, which is routinely used to characterize thin (unpatterned) films, may be employed as a scatterometer if the appropriate algorithms are available. The same would be true of a reflectometer. In some cases, the optical instrument portion of a scatterometer may be specifically designed for scatterometry. In what follows, “SE” is used to designate a spectroscopic ellipsometer used for standard thin film measurements, i.e., film thickness and/or optical properties.
Such measurement instruments are typically employed in stand-alone, off-line fashion, for example, wherein one or more wafers processed by a particular process tool are measured or inspected and a determination is made as to whether measured process parameters (e.g., CDs, overlay registration, film thicknesses, material properties, particle count) are within acceptable limits, and/or whether process related defects are present in the wafers. A stand-alone measurement instrument is not integrated into a process tool, and thus can be used to service multiple process tools. The measurements or inspection may be performed using more than one such measurement instrument, where features are measured using different instruments. Because the measurement instruments are stand-alone systems, the wafers must be transported between the process tool and the measurement instruments before a measurement can be obtained. The stand-alone nature of conventional measurement instrumentation and the resulting transport of wafers between such instruments results in significant down-time in a semiconductor fabrication facility, wherein expensive process tools are shut down pending a final determination as to the existence of problems in the process.
In addition, where wafers must be measured in two or more successive measurement systems in serial fashion, the measurement instrument having the lowest wafer throughput capacity becomes a bottleneck for the inspection process, thus further exacerbating process down-time. Moreover, existing measurement or inspection instruments for semiconductor wafer fabrication processes may provide different results for measurement of the same feature, wherein one instrument may identify a dimensional problem associated with a particular feature, while another such instrument may not. Thus, there is a need for improved measurement systems and methodologies which provide for timely, consistent feature measurement and inspection for wafers being processed in a fabrication facility, and which reduce or mitigate process down-time.